1. Field of the Invention
The present invention relates to a structure of a reflection-type liquid crystal display device, more specifically, to a reflection-type color liquid crystal display device containing a color filter therein, capable of displaying in multiple colors.
2. Description of the Related Art
As a conventional reflection-type liquid crystal display device, a reflection-type liquid crystal display device of a monochrome display using a TN (twisted nematic) liquid crystal cell or an STN (super twisted nematic) liquid crystal cell, is mainly used.
For the growing demand of displaying in colors in recent years, reflection-type color liquid crystal display devices containing color filters therein have been vigorously developed.
The reflection-type color liquid crystal display devices containing color filters therein are broadly classified into the following three types.
The first example is a reflection-type color liquid crystal display device using no polarizing films. There are several types belonging to this example: one type using guest-host liquid crystal in which a black dye is mixed in a liquid crystal material filled in a liquid crystal cell; another type using polymer-dispersion liquid crystal in which a liquid crystal material is dispersed in a high polymer; and so on. Since any one of them does not use a polarizing film, it is excellent in brightness but low in contrast, thus it has not been realized for practical use yet.
The reflection-type color liquid crystal display device using the guest-host liquid crystal is disclosed in, for example, Japan Patent Laid-open No. Sho 59-198489. The reflection-type color liquid crystal display device using the polymer dispersion liquid crystal is disclosed in, for example, Japan Patent Laid-open No. Hei 5-241143.
The second example is a reflection-type color liquid crystal display device using one polarizing film and containing a reflector inside a liquid crystal cell. Moreover, this example has two types. These are one type using an intra-cellular reflector having a mirror surface and a diffusing layer provided on the surface of the liquid crystal cell, and the one using a reflector with scattering properties in the reflecting surface thereof.
Since either type uses only one polarizing film, it is also excellent in brightness but low in contrast.
In the type using an intra-cellular reflector having a mirror surface, though it is bright in a direction of regular reflection of incident light, it becomes abruptly darker at other angles, that is, the viewing angle characteristic thereof is quite poor. In the type using a reflector with scattering properties in the reflecting surface thereof, it is difficult to control scattering properties, thus the manufacturing process becomes complicated.
This reflection-type color liquid crystal display device using one polarizing film is disclosed in, for example, Japan Patent Laid-open No. Hei 3-223715.
The third example is a reflection-type color liquid crystal display device using two polarizing films and having a color filter provided in a liquid crystal cell of a typical monochrome liquid crystal display device. Since this example uses two polarizing films, it is excellent in contrast, but it has a disadvantage of a dark display. However, a reflection-type polarizing film is used for a lower polarizing film, thereby improving brightness, and this reflection-type color liquid crystal display device is considered for practical use.
The reflection-type color liquid crystal display device using a reflection-type polarizing film is disclosed in, for example, Japan Patent Laid-open No. Hei 10-3078.
Hereinafter, the conventional reflection-type color liquid crystal display device using the above reflection-type polarizing film will be briefly explained using FIG. 6.
FIG. 6 shows only a liquid crystal cell 20 of the above reflection-type color liquid crystal display device. The liquid crystal cell 20 includes a first substrate 1 which is a transparent glass substrate and has first electrodes 5, a second substrate 2 which is a transparent glass substrate and has second electrodes 6, and a liquid crystal layer 7 sandwiched therebetween.
A color filter 3 in which three color filters of red (R), green (G) and blue (B) are arranged in alternate order and a protective film 4 are formed on the inner surface side of the second substrate 2, and the second electrodes 6 are formed on the protective film 4.
Both of the first electrodes 5 and the second electrodes 6 are transparent electrodes made of indium tin oxide (ITO). A large number of them are arranged side by side in directions orthogonal to each other to form display pixels at respective intersections thereof. Each color filter of the color filter 3 is arranged on each display pixel in such order of R, G, and B in the both directions orthogonal to each other.
A typical polarizing film (an absorption-type polarizing film) is provided on the visible side (the upper side in FIG. 6) of the liquid crystal cell 20, and a reflection-type polarizing film is provided on the other side (the lower side in FIG. 6), but the illustration thereof is omitted.
Against the liquid crystal cell 20 structured as above, incident light 32 is incident from the upper side of the visible side and then passes through the second substrate 2, the color filter 3, the protective film 4, the second electrodes 6, the liquid crystal layer 7, the first electrodes 5 and the first substrate 1 in order. Thereafter, the light is selectively reflected by the reflection-type polarizing film (not shown) arranged under the first substrate 1 according to the linearly polarized light direction (varying according to the existence of the voltage application between the first and second electrodes 5, 6) of the passed light. However, in this explanation, the passed light is assumed to be reflected by a reflecting surface 31a provided on the lower surface of the first substrate 1 for convenience.
Reflected light 33a of the passed light returns in the reverse order to be emitted toward the visible side (the upper side), and finally reaches the observer""s eye. At this time, the incident light 32 and the reflected light 33a pass through the color filter 3, thereby displaying in color.
However, if the first substrate 1 disposed at the lower side is thick, as shown FIG. 6, the light 33a reflected by the reflecting surface 31a passes through a different color filter of other pixel from that the incident light 32 passes. Accordingly, in this case, a color image is displayed only in poor chroma because of occurrence of mixture of colors.
When the first substrate 1 at the lower side is made thin, the reflecting surface 31a takes a position, for example, shown by a virtual line in FIG. 6. In this case, the incident light 32 and reflected light 33b pass through the same color (blue in the example of the illustration) filter of the same pixel. Consequently, an excellent color image can be displayed without reduction in chroma.
As the pitch of the display pixel becomes smaller or the incident light tilts more, a phenomenon of reduction in chroma becomes increasingly apparent. For this reason, the reflecting surface 31a is required to be disposed closer to the liquid crystal layer 7.
The distance between the reflecting surface 31a and the liquid crystal layer 7 is almost determined by the thickness of the first substrate 1. Accordingly, in order to make the reflecting surface 31a as close as possible to the liquid crystal layer 7, a substrate as thin as possible is preferably used. Thereby, the influence by adjacent pixels decreases, and it becomes possible to display a clear image.
In other words, in order to display an image excellent in chroma at wider viewing angle, it is better that the thickness of the first substrate 1 is made as thin as possible.
However, when a liquid crystal cell is actually manufactured using a thin substrate, the strength of the substrate decreases as the thickness thereof becomes thinner. Accordingly, the frequency of occurrence of defects due to damage of the substrate or the like increases, it becomes difficult to securely manufacture a durable liquid crystal cell.
Moreover, when securely providing a high-definition liquid crystal display device, a method in which a semiconductor integrated circuit device (hereinafter referred to as xe2x80x9ca driving ICxe2x80x9d) containing a liquid crystal driving circuit therein is directly bonded on a substrate and an image display is performed using the driving IC, the so-called chip-on-glass method, is used. When the chip-on-glass is employed, it is difficult to securely bond the diving IC unless the substrate has certain strength.
An object of the present invention is to solve the above problems and to securely provide a reflection-type color liquid crystal display device capable of performing a color image display bright and excellent in chroma.
To achieve the above object, the present invention provides a reflection-type color liquid crystal display device structured as follows.
A liquid crystal cell is structured by sandwiching a liquid crystal layer made of nematic liquid crystal which is aligned at a twist angle of 180xc2x0 to 270xc2x0 between a first transparent substrate having first electrodes and a second transparent substrate having second electrodes, and providing a color filter of a plurality of colors on at least one substrate of said first and second substrates.
Moreover, a retardation film and a polarizing film are provided outside the second substrate of the liquid crystal cell in order, and a diffusing layer, a reflection-type polarizing film and a light absorbing layer are provided outside the first substrate in order.
Furthermore, the first substrate and the second substrate of the liquid crystal cell are bonded together with an anisotropic conductive sealant that is anisotropic in electrical conduction direction. The first substrate is made thinner in thickness than the second substrate, and a liquid crystal driving integrated circuit (a driving IC) is mounted on the thicker second substrate.
The first electrodes on the first substrate and wiring patterns formed on the second substrate and connected to the liquid crystal driving integrated circuit are individually and electrically connected through the anisotropic conductive sealant.
Through the structure as described above, the first substrate can be made thin while the strength of the liquid crystal cell is kept by the thickness of the second substrate, and the second substrate has a sufficient thickness and strength for manufacturing, thus the liquid crystal cell can be securely manufactured.
In addition, since the liquid crystal driving IC is also mounted on the second substrate having a sufficient thickness and strength, it can be directly bonded to the substrate to be mounted, and it can easily connect to the first electrodes on the first substrate through the anisotropic conductive sealant.
Incidentally, if the color filter is provided on the second substrate having a sufficient thickness, strength and accuracy, even a high definition color filter can be securely manufactured with high accuracy.
Moreover, the first electrodes are made thinner or higher in optical transmittance than the second electrodes, thereby a brighter display can be performed.
The above and other objects, features and advantages of the invention will be apparent from the following detailed description which is to be read in conjunction with the accompanying drawings.